118 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
main,
about N. 35° E. and dips 50° SE. In both megascopic and microscopic
appearance it is practically identical with the lighter phases of the
schist from the road-metal quarry in Brunswick village (p. 61). As at
that quarry, dominant acidic bands of schist, prevailingly pink or gray
in tone, alternate with smaller amounts of dark-gray bands of quartz
diorite and other nearly black bands of diorite schist. Under the
microscope these schists show no cataclastic structures; they owe their
foliated structure to parallel elongation of the hornblende grains and
to some extent also of the grains of biotite and quartz. Nothing either
in their texture or their composition indicates that they are not
primary-flow gneisses.
Both
in the lighter and darker phases of the schist, but much more
abundantly in the lighter, are coarser bands of pegmatitic texture,
consisting mainly of quartz and feldspar, with some biotite and
magnetite. Many of these are parallel to the foliation of the schist
and are of even width and uniform character for several yards. Others,
especially the larger masses, cut distinctly across the schist folia,
the contact being sharp and without suggestion of absorption.
An
interesting feature of some of the pegmatite bands which parallel the
foliation of the schists is the presence in them of a slight foliation
parallel to that of the inclosing schist. As in the schist, this
foliation is defined by bands richer than the bordering portions in
hornblende and biotite. In one place a faint foliation is perceptible
in the center of a pegmatite mass 1-1/2 feet wide that cuts across the
foliation of the schists. It does not parallel the trend of the dike
but does parallel the foliation of the inclosing schist and is defined
by the arrangement of the quartz in elongate and somewhat irregular
bands. As there is no evidence of appreciable absorption of the schist
by the pegmatite magma, and also no evidence of metamor-phism
subsequent to the intrusion of the pegmatite, such foliation in the
pegmatite is strongly suggestive of parallel flowing movements in the
schist and in some of the pegmatite. The field and microscopic evidence
on the whole favors the conception that the schists are of primary or
flow-igneous origin, and that some of the pegmatite was crystallizing
before flowage had entirely ceased in the bordering schist, but that
other portions of the pegmatite were intruded after the schist had
completely solidified. The practical identity in mineral character
between the different masses of pegmatite at this quarry suggests that
the distinctly intrusive portions were only slightly later
crystallizations than their host and that all the pegmatite had the
same magmatic source.
One
of the largest masses of graphic granite observed was on the west slope
of the 180-foot hill in the sharp bend of Androscoggin River just west
of Brunswick. The ledge, which is in plain sight from the railroad
track, is 150 feet long and averages 25 feet wide.
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